1. Field
This disclosure relates generally to semiconductor packages, and more specifically, to a molded semiconductor package having a filler material.
2. Related Art
Typically semiconductor packages have a die attached to a substrate. Bond pads located on the die are connected using wires to bond posts located on the substrate. Increasingly, semiconductor packages include a large number of wires because of higher pin count. In addition, certain semiconductor packages may include multi-tier pad rings for connecting wires to the die. As a result, these semiconductor packages have a high wire density. In particular, in some instances the wires may be so close to each other—particularly near the die—that the wire density may exceed 20 wires per millimeter of die edge.
The die and the wires connecting the bond pads to the bond posts are molded over using a mold compound. To ensure reliability of the semiconductor package, the mold compound is selected in a manner that its coefficient of thermal expansion (CTE) is as close as possible to the CTE of the substrate and the die. To achieve this objective, typically mold compound includes a combination of a resin and a filler material. The filler material is carefully selected to ensure that the CTE of the combination of the resin and the filler material is as close as possible to the CTE of the substrate and the die. Filler material may have particles that vary in size from 5 microns to 70 microns. As part of the molding process, the mold compound, including the filler material, is flowed across a top surface of the die contained in the semiconductor package.
As noted above, increasingly the semiconductor packages have a high wire density, which in turn results in screening out of the filler particles in certain regions of the molded semiconductor package. In particular, by way of example, the regions of mold compound near the die edges may have filler material screened out. This causes the CTE of such areas to be higher than the CTE of the die and the substrate. Such mismatch in the CTE of these areas and the die and the substrate makes these areas prone to developing cracks. Such cracks can degrade the reliability of the semiconductor packages. By way of example, a crack formed in the molded package area near the die may transfer enough stress to a joint between the bond pad and the ball bond that the wire may be electrically de-coupled from the bond pad. This in turn can cause the molded semiconductor package to become dysfunctional.
Accordingly, there is a need for a molded semiconductor package that is more reliable.